A heat exchanger is an apparatus for exchanging heat between a fluid, usually one at a high temperature and one at a low temperature. Charge air coolers are specific heat exchangers that are used in particularly stressful environments, such as on internal combustion engines with turbochargers or superchargers.
A turbocharger includes a turbine wheel that is driven by exhaust gases from an engine, and which drives a rotary compressor. A supercharger includes a rotary compressor which is driven by an engine or by a motor that is powered by the engine. Both devices permit an increase in power without adding additional cylinders or substantially increasing the size of the engine. The rotary compressors compress the air entering the engine to permit more air and fuel to enter the cylinders. Compressing the air raises the pressure in the system, which in turn raises the temperature.
When the air is compressed by the turbocharger or supercharger, it is also heated, which causes its density to decrease. In a charge air cooler, the hot combustion air from the turbocharger or supercharger passes through the cooler and into the engine. Ambient air also passes through the charge air cooler separately from the combustion air—often blown across the outside of the air cooler—and acts as the cooling fluid in the heat exchange process. By cooling the combustion air prior to sending it into the engine, the density of the air increases which permits more air to enter the engine and increases the power and efficiency of the engine.
Charge air coolers are not limited to use with turbocharged or supercharged engines, but may also be used with other engines where the pressure and temperature are elevated, such as diesel engines. While an automotive engine is one application for the charge air cooler, it also can be used in other types of engines.
Currently, charge air coolers are typically made of aluminum and operate at temperatures below about 250° C. Newer engines are being designed to improve efficiency and decrease emissions by increasing the boost pressure thus the new charge air coolers will be operating at temperatures of 250° C.–300° C. and higher. The yield strength of aluminum drops quickly as temperatures increase above 150° C., and typically becomes too weak for use in these applications at about 250° C. One multi-tube heat exchanger is disclosed in EP0805331 where the tubes are formed of round aluminum or aluminum alloy. Such a construction will most likely fail at high temperatures by rupture since the aluminum tubes will be weakened by the high heat conditions.
Charge air coolers currently operate at pressures of less than about 3 bars and use flat, wide tubes to transport charge air from the turbocharger compressor. The flat tubes contain internal fins brazed to the inner walls of the tubes to facilitate heat transfer. The internal fins also act as support for the tube under higher pressures to prevent the tube from becoming round. Any flaws or inconsistencies in the brazed joined within the tube will result in failures as pressures near 3 bars. To meet future emission guidelines, newer charge air coolers will be required to operate at pressures of from about 3 to 10 bars and even above 10 bars up to about 40 bars. Current designs of charge air coolers would require heavy gauge materials to operate at these pressures. The heavier gauge materials increase the weight and cost of the components and also increase the pressure drop of the air traveling through the tubes. The use of such heavy gauge materials is unacceptable for these reasons so alternative constructions need to be considered.
An example of a heat exchanger for use in high pressure refrigeration systems is disclosed in US 2003/0102116. The heat exchanger in this application is made of aluminum, and as such, it will not withstand temperatures above 250° C. at temperatures necessary for use in a charge air cooler, due to the low strength of aluminum at such temperatures.
U.S. Pat. No. 6,470,964 discloses a heat exchanger tube for use in the condenser of an air conditioner or refrigerator. The tube is capable of withstanding moderately high operating pressures by virtue of connected depressions on opposite sides of a flat tube. At pressures of about 40 bars, it is unlikely that the tube will maintain its flat shape.
U.S. Pat. No. 6,182,743 discloses a heat exchanger tube having an internal surface that is configured to enhance the heat transfer performance of the tube. The internal enhancement has a plurality of polyhedrons extending from the inner wall of the tubing. The polyhedrons have first and second planar faces disposed substantially parallel to the polyhedral axis. The polyhedrons have third and fourth faces disposed at an angle oblique to the longitudinal axis of the tube. The resulting surface increases the internal surface area of the tube and the turbulence characteristics of the surface, and thus, increases the heat transfer performance of the tube. The high pressure capabilities of such tubes are not discussed. This tube is used in air conditioning and refrigeration systems units having refrigerant flowing inside these tubes. The refrigerant changes phase from gas to liquid in the condenser heat exchanger part of the system and from liquid to gas in the evaporator heat exchanger part of the system.
Due to the low temperatures required to operate with aluminum, some applications use a pre-cooler to cool the air in separate stages. The hot air is pre-cooled in the first stage and later cooled in the aluminum charge air cooler. Such a system is more complex than the present invention and adds to the weight, size, and cost of the system.
While other metals and metal alloys can be considered for high pressure, high temperature applications, most do not have the high heat transfer properties of copper or copper alloys. While it is known that heat exchanger tubes made of steel, stainless steel or nickel base alloys have much greater temperature and pressure resistance, such tubes are more expensive than copper and are not as efficient or effective in transferring heat. In addition, such other metals and alloys would add significantly to the weight and cost of the system.
Accordingly, there is a need for an improved charge air cooler that is capable of withstanding high pressures and high temperatures, as currently used and as expected in the future. The present invention now provides an improved construction for use in such applications.